1. Field of Invention
This invention relates to optical filters. Specifically, the present invention relates to devices for transmitting light of certain wavelengths and blocking light of other wavelengths.
2. Description of the Related Art
Multispectral bandpass optical filters are used in a variety of demanding applications ranging from laser protection glasses to virtual reality imaging applications. Such applications require reliable, durable filters that accurately transmit light of specific wavelengths.
A conventional discrete layer rejection filter includes a stack of optical films, i.e., layers. The indices of refraction of the layers alternate between high and low levels. The alternating indices have a resonating effect on an input light beam resulting in one or more reflection bands, i.e., rejection lines. (A rejection line or reflection band is a portion of the spectrum of the input light that is reflected and not transmitted.)
The stack of layers is called a quarter-wave stack as each layer has an optical thickness of .lambda.hd o/4 where .lambda..sub.o is the wavelength of light reflected due to interference resonance. An interference resonance is also produced for input light having wavelengths .lambda..sub.i, where .lambda..sub.i =.lambda..sub.o /m for odd values of m. These harmonics result in an undesirable family of rejection lines at progressively shorter wavelengths. In addition, interfaces between the multiple layers are prone to mechanical failure when exposed to high temperatures or mechanical stresses.
In the conventional discrete layer rejection filter, multiple transmission bands, i.e., passbands, occur between strategically placed reflection bands. The reflection bands are placed so that the passbands are centered at the desired passband frequencies. Several quarter wave stacks having different resonant wavelengths are stacked in series to produce multiple passbands between the reflection bands. However, due to the creation of undesirable reflection lines at .lambda..sub.o /m wavelengths, applications requiring extended passbands at wavelengths shorter than the reflection bands are difficult to address. Also, accurate positioning of the multiple passbands is exceedingly difficult due to manufacturing limitations. In addition, as the number and width of the rejection bands increase, so does the thickness and unreliability of the filters. Mechanical stresses at the layer boundaries increase with layer thickness. To increase filter durability, special optical layer materials are used to balance compressive and tensile stresses. This increases the price and decreases the design flexibility of the filters. Still, when exposed to high power lasers, such filters tend to de-laminate at the interfaces between the layers due to differences in thermal expansion coefficients of the layer materials.
A filter that addresses problems associated with very thick optical filters is the induced transmission filter having a thin metallic film. The metallic film is impedance matched with films that are deposited on both sides of the metallic film. The metallic film with the impedance matched layers is then placed between quarter-wave stacks, each layer of the stacks having an optical thickness of .lambda..sub.o /4. This results in a passband centered at .lambda..sub.o. Dispersion properties of the metallic layer prevent harmonics from forming. However, these filters only operate at a single bandpass wavelength and have relatively poor transmission efficiency.
In another approach known as the rugate approach, the discrete layers used in conventional discrete layer filters are replaced by a continuous layer having an index of refraction that varies through the layer. The index of refraction is varied to produce a profile with the desired optical properties. Such rugate filters are relatively sturdy as the number of abrupt material interfaces are minimized.
To produce broadband filters, i.e., filters having wide rejection bands, two separate rugate filters are stacked together. The edges of the reflection bands of the filters are positioned above and below the desired center of the passband of the broadband filter. Such filters require careful phase-matching of the constituent transmission lines to obtain good transmission in the passband. This can be an expensive and painstaking process. Also, as the passband narrows, successful placement and shaping of the passband becomes increasingly difficult.
A variable period broadband design is disclosed in U.S. Pat. No. 5,475,531 issued Dec. 12, 1995 to A. Turner and T. Rahmlow and entitled BROADBAND RUGATE FILTER, the teachings of which are herein incorporated by reference. In this filter, the frequency of the variation of the index of refraction of the rugates is continuously varied to create the desired optical properties. Narrow bandpass filters are constructed by inducing phase discontinuities in the refractive index profile at points corresponding to the wavelengths at which transmission is desired. This approach however, requires accurate placement of the phase discontinuity which may be difficult in some applications.
Hence, a need exists in the art for a thin, durable filter for selectively transmitting light of multiple specific wavelengths. There is a further need for an accompanying method for accurately and efficiently controlling the placement and width of the passbands.